Many firearms, including the M-16, M-4 and AR-15 rifles, use a direct gas system for ejecting a spent cartridge, resetting a trigger mechanism, and cycling a new round for firing. The direct gas system utilizes a tube that has an inlet port in the barrel of the gun such that when a bullet passes the inlet port combustion or propellant gasses enter the gas tube and are directed back to the bolt carrier to cycle the action for the firing of another round. The use of combustion or propellant gasses results in significant quantities of propellant residue being deposited in and on the bolt carrier as well as in the action. The combustion or propellant gasses fill the interior of the action after the carrier key is displaced away from the end of the gas tube, resulting in propellant or combustion gasses being distributed throughout the upper and lower receivers. As a result, combustion or propellant gas residue builds up on the bore of the bolt carrier. Additionally, considerable heat is transferred to the upper and lower receivers as well as the bolt and the bolt carrier and related parts. In an effort to prevent the build up and corrosion that results from the combustion of propellant gasses, the bolt carrier and bolt are usually heavily lubricated. When the firearm is used in dirty or sandy conditions, fine particles of dirt or sand may adhere to the lubricant, creating abrasive slurry and resulting in jamming of the action and possible misfires. Additionally, heat buildup may cause parts to prematurely fatigue and fail. While these direct gas systems for recycling a round for automatic firing are relatively simple, they often require frequent and meticulous maintenance so as to maintain the firearm in a reliable condition and prevent jamming or misfiring.
Efforts have been made to improve on the direct gas system. One such system is a gas tappet system having a cylinder and piston with a connected operating rod to move the bolt carrier. These prior gas tappet systems often require extensive modification of an existing firearm to install. For example, often the small hole in the upper receiver that permits passage of the gas tube in a direct gas system has to be modified and substantially enlarged so as to accommodate the much larger diameter piston that is necessary in a gas tappet system. Accordingly, once modified, it is difficult or impossible to remove the gas tappet system and reinstall the original direct gas system into the firearm. Additionally, many prior gas tappet systems utilize a short stroke piston and operating rod that rely on a large impact force and inertia to move a bolt and bolt carrier. Since the operating rod often impacts the bolt carrier at the top of the bolt carrier and not along a center line, the relatively large impact force on the bolt carrier causes torque or a twisting motion which causes the rear of the bolt carrier and the guide rails at the front of the bolt carrier to dig into the upper receiver often resulting in impact damage. This impact damage causes excessive wear and jamming. Additionally, these prior gas tappet systems often utilize a gas cylinder and piston having a coaxially attached operating rod coaxially that often results in the gas cylinder being taller than the original gas tube in a direct gas system. Therefore, often further modifications of the firearm are required. These further modifications often require new hand guards and sometimes replacement of the front sight base which makes the modification to the firearm expensive and complicated and often requires sophisticated tooling and equipment.
Therefore, there is a need for a gas tappet system that can be easily installed into M-16, M-4 and AR-15 type rifles without any or limited modification to the rifle. Additionally, there is a need for the design to substantially reduce the impact force applied to the bolt carrier and therefore reduce or eliminate the excessive torque or twisting that can cause excessive wear and possible jamming.